In a gas turbine engine, ambient air is drawn into a compressor section. Alternate rows of stationary and rotating aerofoil blades are arranged around a common axis, together these accelerate and compress the incoming air. A rotating shaft drives the rotating blades. Compressed air is delivered to a combustor section where it is mixed with fuel and ignited. Ignition causes rapid expansion of the fuel/air mix which is directed in part to propel a body carrying the engine and in another part to drive rotation of a series of turbines arranged downstream of the combustor. The turbines share rotor shafts in common with the rotating blades of the compressor and work, through the shaft, to drive rotation of the compressor blades.
It is well known that the operating efficiency of a gas turbine engine is improved by increasing the operating temperature. The ability to optimise efficiency through increased temperatures is restricted by changes in behaviour of materials used in the engine components at elevated temperatures which, amongst other things, can impact upon the mechanical strength of the blades and a rotor disc which carries the blades. This problem is addressed by providing a flow of coolant through and/or over the turbine rotor disc and blades.
It is known to take off a portion of the air output from the compressor (which is not subjected to ignition in the combustor and so is relatively cooler) and feed this to surfaces in the turbine section which are likely to suffer damage from excessive heat. Typically the cooling air is delivered adjacent the rim of the turbine disc and directed to a port which enters the turbine blade body and is distributed through the blade, typically by means of a labyrinth of channels extending through the blade body.
Turbine blades are known to be manufactured by casting methods. A mould defines an external geometry of the turbine and a core is inserted into the mould to define the internal geometry, molten material (typically a ferrous or non-ferrous alloy) is then cast between the mould and the core and the core subsequently is removed, for example by leaching.
The core geometry defines cooling passages through which cooling air will flow within the blade body. One known means of increasing the convective cooling effectiveness of these cooling passages is to use pins or pedestals within the passages. These pedestals or pins increase the wetted area of the main cooling passages allowing more heat transfer from a main cooling passage internal face to cooling air passing through the main cooling passage. These pins or pedestals can also be used to increase pressure loss in a main cooling passage. Appropriate arrangements can serve to control the rate of cooling air leaving the main passages.
A core is injection moulded using a core die. This die represents the geometry of the blade to be cast around the core. Pedestal arrangements optimised for cooling performance, particularly when designed to achieve relatively large pressure losses, are difficult to manufacture. In manufacturing the core, it must be possible to fill the core die with core ceramic. If the pedestal arrangement has been designed solely to achieve large pressure losses in the cooling gas flow in the finished blade, it will have the same effect on the flow of ceramic into the core die. A possible consequence is poor core die fill in that area and improperly formed pedestals or passages. If the core die is not properly filled, the resulting core will not be suitable for casting a blade to the desired geometry.
In known prior art arrangements, the trailing edge of a turbine blade body is provided with one or more arrays of pins or pedestals closely aligned in parallel rows. The rows may be slightly staggered to allow the adjacent rows to be placed as close together as possible whilst maintaining structural integrity around the pins/pedestals. Dimensions of the pedestals and spacing between them are in the order of millimetres, even fractions of millimetres. Whilst such an arrangement is effective at trailing edge cooling, it presents a considerable challenge when manufacturing a core defining this internal geometry.